The present invention relates to ink compositions suitable for thermal ink-jet printing, and, more particularly, to ink compositions for preventing kogation and prolonging resistor life in ink-jet pens.
The use of ink-jet printing systems has grown dramatically in recent years. This growth may be attributed to substantial improvements in print resolution and overall print quality coupled with appreciable reduction in cost. Today""s ink-jet printers offer acceptable print quality for many commercial, business, and household applications at costs fully an order of magnitude lower than comparable products available just a few years ago. Notwithstanding their recent success, intensive research and development efforts continue toward improving ink-jet print quality, while further lowering cost to the consumer.
An ink-jet image is formed when a precise pattern of dots is ejected from a drop-generating device known as a xe2x80x9cprintheadxe2x80x9d onto a printing medium. The typical ink-jet printhead has an array of precisely formed nozzles located on a nozzle plate and attached to an ink-jet printhead substrate. The substrate incorporates an array of firing chambers that receive liquid ink (colorants dissolved or dispersed in a solvent) through fluid communication with one or more ink reservoirs. Each chamber has a thin-film resistor, known as a xe2x80x9cfiring resistor,xe2x80x9d located opposite the nozzle so ink can collect between the firing resistor and the nozzle. In particular, each resistor element, which is typically a pad of a resistive material, measures about 35 xcexcmxc3x9735 xcexcm. The printhead is held and protected by an outer packaging referred to as a print cartridge, i.e., ink-jet pen.
Upon energizing of a particular resistor element, a droplet of ink is expelled through the nozzle toward the print medium, whether paper, transparent film or the like. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements, thereby forming alphanumeric and other characters on the print medium.
The small length scale of the nozzles, typically 10 to 40 xcexcm in diameter, require that the ink not clog the nozzles. Further, repeated firings of the resistor elements that must withstand many millions of firings over the life of the ink cartridge to be commercially practical, can result in fouling of the resistor elements and degrading pen performance. This build up of residue on the resistor elements is unique to thermal ink-jet printers and is known as kogation and defined as the build-up of residue (koga) on the resistor surface.
Besides the problem of kogation, thermal ink-jet printer resistor surfaces are susceptible to passivation layer damage by cavitation, contamination and many other sources. Such passivation layer damage literally results in microscopic holes on the resistor surface which significantly shorten resistor life.
Customer and profit demands require smaller drop volumes, color-laser-like ink permanence, and xe2x80x9cpermanentxe2x80x9d print heads. Smaller drop volumes give better spatial and chroma resolutions. However, kogation appears to be worse in smaller drop volume pens. Smaller drop volumes also means that each resistor must fire a greater number of times to transfer the same amount of ink to the page. The greater number of firings required of the resistor results in more passivation layer damage.
In spite of the increased pressure on kogation and passivation layer damage, the trend is towards longer print-head life, using pens with replaceable ink supplies such as (but not limited to) off-axis ink reservoirs that are connected to the pens by hoses and ink reservoirs that snap onto the print head. Infrequent need for replacement of the print heads with prolonged resistor life reduces the cost and servicing required of the customer. High-speed, high-throughput photocopier-like products that may be envisioned for the future will greatly increase ink usage and will most likely greatly push further the demands on print-head life. With higher pen-to-paper relative speeds, high-throughput products will be more sensitive to kogation induced drop velocity variations.
It has been previously disclosed that oxyanions, especially phosphates, reduce kogation. The mechanism was attributed to the additive eliminating or reducing adsorption of dye and/or decomposition products onto the resistor. Furthermore, the phosphate esters disclosed for kogation control are short chain phosphate esters, not long-chained, surface active phosphate esters as in the present invention. It has also been disclosed that organic acid sulfonate, such as sodium methane sulfonate, and bile salt (e.g., sodium cholate) additives can be used for reducing kogation. Other disclosed ways of achieving kogation control are, for example, macrocyclic polyethers to complex cations in ink-jet ink compositions; isopropanol/water rinse to remove phosphate antistatic material from ink foam; and removing koga by various mechnical means such as applying pressure to the channels of the ink-jet pen or dry-firing the resistor. Recently, it has been disclosed that phytic acid in ink-jet ink can be used to reduce foreign matter deposits on a surface of an ink-jet heating head.
The use of phosphate ester in ink-jet inks has also been disclosed. However, such disclosures only show the general utility of phosphate esters to produce a surfactant effect, such as for improved water-fastness, and do not propose phosphate esters as a solution to the problem of kogation or passivation layer damage.
The use of ethylenediaminetetraacetic acid (EDTA) to capture calcium for improving the problem of crusting/decap (i.e., formation of ink precipitate either in the pen nozzle or immediately outside the pen nozzle due to exposure of ink to air) in ink-jet inks has also been disclosed. Furthermore, above 100 ppm EDTA was found to erode away the resistor coating, while a slight amount of EDTA was asserted to have the positive effect of preventing a large amount of the koga from adhering to the resistor surface, thus maintaining the resistor surface in a smooth state.
Nitrate and phosphate ions have been used in ink-jet ink compositions to prevent kogation. In ink-jet pens currently on the market, these two solutions have proved to have little benefit and substantial drawbacks. Both nitrate and phosphate ions have potential health and environmental complications and phosphate ion has been shown to etch the tantalum surfaces of resistors in ink-jet pens. Something is therefore needed to reduce kogation on ink-jet resistors which does not have the negative effects that accompany nitrate and phosphate ions.
Even though some kogation and/or passivation layer damage control methods in ink-jet ink pens are known, all of them are either limited in their effectiveness, are not economically feasible or have undesirable side effects for pens needing long resistor life. Thus, there is even more of a need to find a way to effectively deal with the problem of kogation and passivation layer damage on ink-jet resistors.
The present invention relates to an ink-jet ink composition comprising at least one colorant; and an aqueous vehicle, the vehicle comprising at least one refractory or noble metal-reactive component in an amount sufficient, when the composition is used in an ink-jet pen, to form a protective thin layer on an outer layer of a resistor surface of the ink-jet pen, the outer layer comprising a refractory or noble metal, the refractory or noble metal being selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, gold, silver and platinum.
The present invention relates to a method for ink-jet printing, said method comprising the step of ejecting ink, said ink comprising at least one colorant; and an aqueous vehicle, the vehicle comprising at least one refractory or noble metal-reactive component in an amount sufficient, when the composition is used in an ink-jet pen, to form a protective thin layer on an outer layer of a resistor surface of the ink-jet pen, the outer layer comprising a refractory or noble metal, the refractory or noble metal being selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, gold, silver and platinum.